Air ducts used in commercial and residential construction are typically fabricated from thin metallic sheets that are cut and folded into shape. Such sheets are light in weight, easy to form, and inexpensive. However, when subjected to changes in temperature and pressure caused by air flowing through them, metallic ducts tend to expand and contract. Expansion and contraction of ductwork can be accompanied by unpleasant creaks and, over time, can lead to substantial leaks of air at joints between ducts. To limit ductwork movement, internal bracing is employed to reinforce most ducts of significant size.
Internal bracing is most often provided by extending rods, often referred to as “conduits,” from the top to the bottom of a duct and from one side of a duct to another. Usually, the rods are outfitted with threaded sockets at their opposed ends that are aligned with small holes drilled in a duct. Bolts, extended through the holes into the sockets from the exteriors of ducts, secure the rods in place. The process of aligning the rods with the holes and tightening the fasteners is tedious, time-consuming, and requires two laborers to complete: one on the interior of the duct to move the rods about and the other on the exterior of the duct to position and tighten the threaded fasteners.
Some have proposed improvements to the standard bracing system. For example, we have for years offered to HVAC contractors rods with threaded studs extending from their opposite ends that one laborer can “snap-fit” into predrilled holes in ducts. After snap fitting, the same laborer can easily screw nuts onto the portions of the threaded studs extending outwardly through the holes in the ducts to secure the rods in place. Sadly, in the hands of unskilled individuals, the studs can sometimes snag on the interiors of ducts and crease ducts in an unsightly manner as the rods are being positioned. To overcome this problem, studs that retract into the ends of rods have been recently proposed. These retractable studs are complex in their construction and require special crimping tools for attachment to a rod in a time-consuming operation.